Since last week, I've been getting a few random emails that appear totally garbled (see print-screen below). From looking at the message source, the only thing in common between them I was able to find was the base64 encoding, so I'm guessing that's somehow related to this problem.

As far as I know, Thunderbird does decode base64 encoding, correct? Any ideas as to why this is happening?

Yes, Thunderbird decodes base64. Select the message and choose View – Message Source to see what is really in the message. Perhaps it is not really encoded, and the Content-Transfer-Encoding header is wrong. Otherwise copy and paste the first couple of lines of the message source in your reply here.

Rod Whiteley wrote:Yes, Thunderbird decodes base64. Select the message and choose View – Message Source to see what is really in the message. Perhaps it is not really encoded, and the Content-Transfer-Encoding header is wrong. Otherwise copy and paste the first couple of lines of the message source in your reply here.

This is getting stranger by the minute... I just realized this only happens on emails which are replies to (some of) our emails. And on top of that, I just sent an email to a person with which I had this problem (of the reply showing up all garbled), and her reply to my last email is displayed correctly. I'm totally clueless as to what is happening here.

A "security" program intercepted the message and added a line that reads "Bayesian score: 31.39" at the top. This destroys the integrity of the encoded message. The program that intercepted the message has a bug.